Since the recent progress of vacuum degassing molten steel has made the ingot-making of extremely low carbon steel easy, demand for extremely low carbon steel sheets having good formability has further been increasing. Among the steel sheets, those extremely low carbon steel sheets in which Ti and Nb are compositely added and which are disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) Nos. 59-31827 and 59-38337 have extremely good formability, bake hardening (BH) properties and excellent hot dip zinc coating characteristics. Accordingly, the extremely low carbon steel sheets occupy an important position. However, the amount of BH of the steel sheets does not exceed the level of conventional BH steel sheets, and has a drawback that they cannot secure their cold non-aging properties when a further amount of BH is tried to be imparted thereto.
Furthermore, there are many disclosures with regard to extremely low carbon steel sheets excellent in formability and containing neither Ti nor Nb, for example, in Japanese Examined Patent Publication (Kokoku) No. 53-22052, and Japanese Unexamined Patent Publication (Kokai) Nos. 58-136721 and 58 -141335.
On the other hand, many attempts have heretofore been made to increase the strength of steels while the formability thereof is being secured. Particularly, in the case of steels which have a tensile strength from 30 to 50 kgf/mm.sup.2 and to which the present invention relates, the strength thereof has been increased by adding P, Si, etc., to the steels and utilizing these solid solution strengthening mechanisms. For example, Japanese Unexamined Patent Publication (Kokai) Nos. 59-31827 and 59-33837 disclose processes for producing high strength cold rolled steel sheets having a tensile strength of up to a class of 45 kgf/mm.sup.2 by adding mainly Si and P to extremely low carbon steel sheets to which Ti and Nb have been added. Japanese Examined Patent Publication (Kokai) No. 57-57945 disclose a typical prior technique relating to a process for producing a high strength cold rolled steel sheet by adding P to an extremely low carbon steel to which Ti has been added.
Moreover, in extremely low carbon steels containing neither Ti nor Nb, Japanese Examined Patent Publication (Kokoku) No. 58-57492 and Japanese Unexamined Patent Publication (Kokai) No. 58-48636 disclose techniques for highly strengthening steels by adding P, and Japanese Unexamined Patent Publication (Kokai) No. 57-43932 discloses a technique for utilizing Si.
As described above, heretofore P firstly and Si secondly have often been used as strengthening elements for reasons as described below. It has been considered that the addition of small amounts of P and Si increases the strength of a steel because of their very high solid solution strengthening capability, that the addition does not lower the ductility and deep drawability of the steel much, and that the addition cost does not increase much. However, in reality, when strengthening the steel with only these elements is tried, not only the strength but also the yield strength of the steel increases significantly. As a result, defects of surface shapes are sometimes formed, and the use of the steel for automobile panels may sometimes be restricted. Moreover, when the steel is hot dip coated with zinc, Si in the steel may induce the formation of poor coating, and P and Si may markedly lower the alloying rate. Accordingly, the addition of P and Si has a problem that they may lower the productivity of the steel products.
On the other hand, the use of Mn and Cr as solid-solution strengthening elements is also known. Japanese Unexamined Patent Publication (Kokai) Nos. 63-190141 and 64-62240 disclose techniques for adding Fin to Ti-containing extremely low carbon steel sheets, and Japanese Examined Patent Publication (Kokoku) No. 59-42742 and Japanese Examined Patent Publication (Kokoku) No. 57-57945 which is mentioned above disclose techniques for adding Mn and Cr to extremely low carbon steels to which Ti has been added.
Moreover, Japanese Unexamined Patent Publication (Kokai) No. 2-111841 discloses a cold rolled steel sheet and a hot dip zinc-coated steel sheet which have bake hardenability and good formability and which are prepared by adding Mn in an amount from at least 1.5% to less than 3.5% to an extremely low carbon steel prepared by adding Ti. Mn is added in a large amount for the purpose of lowering the Ar3 transformation point leading to the stabilization of the operation of hot rolling and forming a uniform metal structure. Moreover, the patent publication discloses the addition of Cr and V in an amount from 0.2 to up to 1.0% for the purpose of further improving the ductility. However, the invention is not based on the idea that the addition of large amounts of Mn and Cr improves mechanical properties, particularly a balance between the strength and the ductility. Furthermore, the amount of BH does not deviate from the conventional level in the invention, and the invention does not make the bake hardenability which is higher than conventional values compatible with the non-aging properties.
Furthermore, Japanese Unexamined Patent Publication (Kokai) No. 62-40352 discloses a technique for adding Mn to an extremely low carbon steel containing neither Ti nor Nb. However, the invention of the patent publication can be concluded as follows: (i) the addition of Mn and Cr plays only an auxiliary role of P and Si which are principal addition elements, and therefore the cold rolled steel sheet thus obtained has a high yield strength compared with its strength; and (ii) Mn and Cr are not added for purposes other than (i), for example, they are added for the purpose of neither (a) making the structure subsequent to annealing a mixed structure, nor (b) improving the work hardenability, nor (c) imparting BH to the steel, nor (d) improving the secondary formability, nor (e) improving the coating properties of the hot dip zinc coating.
Still furthermore, Japanese Unexamined Patent Publication (Kokai) Nos. 58-48636 and 57-203721 disclose processes for producing cold rolled steel sheets having excellent bake hardenability and deep drawability by adding a large amount of B to extremely low carbon steels to which Ti and Nb have not been added, and annealing at a temperature from 730.degree. C. to the A3 point. However, these cold rolled steel sheets are composed of a ferrite single phase structure as a prerequisite of the invention quite different from the present invention.
In contrast to the steel sheets having a ferrite single phase structure as described above, steel sheets having composite structures are also known. These steel sheets are exemplified by the so-called Dual Phase steel (DP steel) in which a ferrite phase and a martensite phase are present in a mixture and which is prepared by adding alloying elements such as Si, Mn and Cr to a low carbon aluminum-killed steel, and optimizing a continuous annealing temperature and a subsequent cooling rate. Such a DP steel is known to have an extremely low yield ratio (YR) though it has a high strength, and is also known to have non-aging properties and a high BH. However, the DP steel has a drawback that it has a r-value as low as about 1.0 and a poor deep drawability. In addition, processes for producing such cold rolled steel sheets are disclosed in Japanese Examined Patent Publication (Kokoku) No. 53-39368 and Japanese Unexamined Patent Publication (Kokai) Nos. 50-75113 and 51-39524.
In contrast to these composite structure steel sheets prepared from low carbon aluminum-killed steels, Japanese Examined Patent Publication (Kokoku) Nos. 3-2224 and 3-21611 and Japanese Unexamined Patent Publication (Kokai) No. 3-277741 disclose composite structure steel sheets prepared from extremely low carbon steels. In the preparation of these steel sheets, Ti is further compositely added to extremely low carbon steels in addition to large amounts of Nb and B, and a composite structure of a ferrite phase and phases transformed at a low temperature are formed after annealing, whereby cold rolled steel sheets having a high r-value, a high BH, a high ductility and cold non-aging properties are obtained.
Particularly with regard to Japanese Unexamined Patent Publication (Kokai) No. 3-277741, the patent publication discloses a technique for providing a steel sheet having a bake hardenability, non-aging properties and in addition formability wherein a steel prepared by adding Nb, B and Ti, and in addition Mn and Cr to an extremely low carbon steel is annealed at a temperature from at least AC1-50.degree. C. to less than the Ac1 transformation point to form a structure consisting of a composite structure composed of acicular ferrite having a volume ratio of up to 5% and ferrite. However, as a result of detailed investigation by the present inventors, it has become evident that the technique has the following problems. That is, in a composite structure steel sheet having a second phase in an amount of up to 5% by volume, imparting BH to the steel at least at a conventional level, namely, in an amount exceeding 5 kgf/mm.sup.2 is difficult, and when the amount of BH exceeds 5 kgf/mm.sup.2, the YP-El sometimes exceeds 0.2%, securing the non-aging properties of the steel thus being found to be extremely difficult. For example, a Nb-containing steel containing 0.004% of C, 0.01% of Si, 1.5% of Mn, 1.0% of Cr, 0.05% of P, 0.025% of Nb, 0.04% of Al, 0.0025% of N and 0.01% of S has been annealed while the holding temperature has been changed in the range from 840.degree. to 865.degree. C. whereby the volume ratio of the second phase has been changed from 0 to 20%, and the relationship between the amount of BH and the YP-El after artificially aging the steel has been examined. The results thus obtained are shown in FIG. 1. It is clear from FIG. 1 that securing the non-aging properties of the steel becomes difficult when the steel contains the second phase in an amount of up to 5% by volume. This is probably caused by an insufficient density of migrational dislocations introduced into the ferrite because of a small volume ratio of the second phase.
Furthermore, Japanese Unexamined Patent Publication (Kokai) No. 60-197846 discloses a technique for obtaining a steel having such properties as described above by adding a large amount of B to an extremely low carbon steel containing neither Ti nor Nb. However, as a result of intensively examining the technique, the present inventors have found that forming a composite structure by adding such a large amount of B involves problems as described below:
1) A steel containing such a large amount of B does not lower its Ac1 transformation point. As a result, to obtain a composite structure, annealing the steel at an extremely high temperature becomes essential, resulting in troubles such as rupture of the steel sheets during continuous annealing. PA1 2) Since the temperature range of .alpha.+.gamma. of the steel is extremely narrow, the structure may change in the width direction, or the material of the steel may greatly vary, or a composite structure may be formed or may not be formed when the annealing temperature changes in a magnitude of several degrees centigrade. The production of the steel therefore becomes extremely unstable. PA1 3) The addition of B in a large amount brings about the deterioration of the steel ductility. PA1 4) The addition of B in a large amount causes the formation of a defective metallic coating. The steel is therefore inappropriate for a hot dip zinc-coated steel sheet. PA1 5) Moreover, imparting BH in an amount of at least 5 kgf/mm.sup.2 becomes not only difficult, the steel to which BH is imparted in an amount exceeding 5 kgf/mm.sup.2 exhibits a YP-El exceeding 0.2% after artificial aging. As a result, the non-aging properties of the steel are not secured.
As described above, there have been several proposals on composite structure steel sheets in extremely low carbon steels. However, the amount of BH does not deviate from the conventional level at all, and the non-aging properties remain at a level only slightly exceeding the conventional one.